There are various radial-gap PM-reluctance motors available in the market. However, there is no axial-gap PM reluctance motor seen in the market. The rotor structure of an axial-gap motor is a thin disk, which is very different from the rotor of the radial-gap motor.
It is commonly known that a permanent magnet (PM) electric machine has the properties of high efficiency and high power density. By introducing a reluctance path to a PM motor the total torque that includes the PM synchronous torque and the reluctance torque can be increased.
U.S. Pat. No. 4,996,457 issued Feb. 26, 1991, disclosed a high speed permanent magnet (PM) axial gap machine with multiple stators. This machine employed one rotor sandwiched between two stators. This early machine consisted of two supporting non-magnetic annuli, each having an even number of embedded equi-angularly spaced cylindrical magnets held in place against opposite sides of a ferromagnetic flux return plate. In U.S. Pat. No. 5,117,141 issued on May 26, 1992 equi-angularly spaced cylindrical magnets were embedded in a single non-magnetic disk to enable a single rotor to be used by two stators and to allow flux to pass through both stators and the rotor. This allowed more efficient use of magnetic material, which is the most expensive component of a PM motor.
Permanent magnet motors produce a back-emf and torque that depend upon the amount of magnetic material; however, after the back-emf reaches the level of the supply voltage it becomes difficult to drive the motor.
It is desired to make such an axial gap PM machine that will provide increased torque for the same amount of applied energy.
Naito et al., U.S. Pat. Pub. 2004/0135453, discloses an axial gap PM machine with an iron rotor having reluctance poles. This machine is intended for use as a starter motor and generator in a motor vehicle. This machine has a stator on only one side as it is primarily for starting and providing power for accessories in a vehicle. Such a machine is not deemed suitable for application as a traction drive motor in an electric or hybrid vehicle.
It would be desirable to provide a PM and reluctance pole machine that operates not only for startup and a low-level power supply, but also as a machine that is suitable for operating through the full speed range of a vehicle. Such a machine would have a higher power rating and other characteristics which are different from the prior art vehicle generators.
The single-sided stator in the prior art machine provides a magnetic pull on only one side. This does not provide sufficient axial balance under loading conditions encountered by a vehicle traction motor. The single stator design would result in a heavy axial load under such circumstances.
The prior art starter motor/generator with reluctance poles has a high rotor inertia due to the heavy rotor iron disc which is used for a flux return yoke path.